A heat engine is an energy system that performs conversion of thermal energy from an energy source or heat reservoir to mechanical work. A variety of energy sources may be employed to power the heat engine. These energy sources may include, but not limited to, solar energy, nuclear energy, geothermal energy, combustion gas from a combustion chamber, exhaust gas from a diesel engine, gasoline engine, or gas turbine engine, and flue gases and hot fluids from industrial furnaces or processes. According to one classic definition, a heat engine employs a working fluid without the change of chemical composition and works in cycles with the options of having an open-cycle or a closed-cycle configuration.
Unlike a heat engine according to the above classic definition, an internal combustion engine, strictly speaking, does not work in cycles due to the change in chemical composition of the working fluid. Traditionally, however, the operation of an internal combustion engine may be simplified as cycles for the convenience of analysis. In this regard, the chemical energy associated with a fuel is converted into thermal energy through combustion, and the thermal energy released during combustion is absorbed by the compressed working fluid over a certain time period in a cycle. This time period may be measured in terms of crank angle (CA) or drive shaft rotating angle. As a result, the thermal energy released in the combustion and absorbed by the working fluid in a cycle may be expressed by the following relation:
                              Q          c                =                                            ∫                              θ                s                                            θ                e                                      ⁢                                                                                Q                    .                                    c                                ⁡                                  (                  θ                  )                                            ⁢                                                          ⁢                              ⅆ                θ                                              =                                                                      Q                                      .                    _                                                  c                            ⁡                              (                                                      θ                    e                                    -                                      θ                    s                                                  )                                      =                                                            Q                                      .                    _                                                  c                            ⁢                                                          ⁢              Δ              ⁢                                                          ⁢                              θ                c                                                                        (        1        )            wherein {dot over (Q)}c is the instantaneous combustion heat release rate and {dot over ( Q is the average heat release rate over the combustion duration, both having a unit of J/CA, θs is the crank angle at which combustion starts in the combustion chamber, θe, is the crank angle at which the combustion ends, and Δθc is the combustion duration in CA degrees. Due to the explosive nature of combustion in a combustion chamber and a high average heat release rate, {dot over ( Qc, the combustion duration is normally very small, on the order of 30-40 CA.
For a heat engine, the acquisition of the thermal energy by the working fluid from an external heat source is normally through a heat exchanger that facilitates heat transfer from the heat source to the working fluid due to a temperature difference between the heat source and the working fluid. This heat transfer may occur during a time period in a cycle, which could also be measured through a crank angle (or drive shaft rotating angle):
                              Q          HT                =                                            ∫                              θ                1                                            θ                2                                      ⁢                                                                                Q                    .                                    HT                                ⁡                                  (                  θ                  )                                            ⁢                                                          ⁢                              ⅆ                θ                                              =                                                                      Q                                      .                    _                                                  HT                            ⁡                              (                                                      θ                    2                                    -                                      θ                    1                                                  )                                      =                                                            Q                                      .                    _                                                  HT                            ⁢                                                          ⁢              Δ              ⁢                                                          ⁢                              θ                HT                                                                        (        2        )            wherein {dot over (Q)}HT is the instantaneous heat transfer rate and {dot over ( QHT is the average heat transfer rate over the heat transfer duration, both having a unit of J/CA, θ1 is the crank angle at which the heat transfer begins, θ2 is the crank angle at which the heat transfer ends, and ΔθHT is the effective heat transfer duration in degrees of CA.
It is well known that the heat absorbed by the working fluid in a cycle, either Qc or QHT, may predominantly determine the power output of an engine at a given engine speed. To match the amount of heat transfer, QHT, in a heat engine with the amount of heat released, Qc, in an internal combustion engine over a cycle,QHT={dot over ( QHTΔθHT≈Qc={dot over ( QcΔθc  (3)one way is to provide an average heat transfer rate, {dot over ( QHT, having the same order of magnitude as {dot over ( Qc. This may be attained through a heat exchanger having a large heat transfer surface area or a high heat transfer rate per unit surface area (heat flux), which is primarily determined by the heat transfer mechanism between the heat source and working fluid of the heat engine as well as the temperature difference between the heat source and working fluid.
In many applications, however, the effective average heat transfer rate, {dot over ( QHT, in a heat engine may be at least an order of magnitude lower than the average heat release rate of an internal combustion engine, {dot over ( Qc, with a comparable engine size. Thus, another way as shown Eq. (3) is to provide a much longer duration of heat transfer, ΔθHT, than the heat release duration, Δθc, to attain a sufficiently high QHT for building a heat engine that could be practically viable.
The timing of heat transfer or heat release is also an important issue that must be addressed. In an internal combustion engine operating under the principle of an Otto cycle, the timing of the heat release may be easily controlled, and is preferably set near the top dead center in a piston-type combustion engine for a higher power output and a higher thermal efficiency. For a heat engine, however, the timing of the heat transfer through a heat exchanger may be difficult to control, and in many situations, this heat transfer may inevitably take place over the entire cycle not just near the top dead center, due to thermal inertia factors, such as that related to the mass of the heat exchanger walls. Thus, the increased heat transfer duration as mentioned above may be preferably a time period between after the working fluid is substantially compressed to a higher pressure and before the working fluid has substantially expanded, so that a substantially large portion of the heat acquisition from a heat source in a cycle may occur during this time period and the performance of the heat engine may approach that of an Otto cycle.
It is also well known that for an engine operating at a given speed, both the power output and thermal efficiency may depend on the number of strokes per power stroke in a cycle. For a given heat input in a cycle and a given operating speed, a smaller number of strokes per power stroke will have the benefits of increased power output as well as increased thermal efficiency due to a reduced frictional loss. Thus, it is very important that an increase in the duration of heat transfer not result in an increase in the number of strokes per power stroke in the cycle.
As indicated above, a heat engine may share some similarity with a combustion engine. Thus, a heat engine may be constructed based on the structure of a conventional internal combustion engine such as, but not limited to, four-stroke piston combustion engine, two-stroke piston combustion engine, rotary combustion engine, or free piston combustion engine. Both the two-stroke piston engine and rotary engine may be attractive because of their smaller number of strokes per power stroke in a cycle. Additionally, a heat engine may have a larger size and lower mean effective pressure in comparison with an internal combustion engine of comparable power output. Thus, the working fluid of the heat engine may be pressurized, and a heat engine structure that has a smaller volume-to-power ratio and lower mechanical frictional losses would be preferable.
One of such engine structures may be related to a rotary engine, in particular rotary lobed combustion engine, such as the Wankel rotary engine. In addition to the standard Wankel rotary engine structure as being known today, Wankel's study on rotor and housing configurations covered a range of shapes from a two-lobed rotor in an ovoid-like housing up to a four lobed rotor in a three lobed housing. His original rotary engines were DKM series, in which unlike the standard Wankel rotary engine, the outer rotor is the driven member and turns three times for every two turns of the inner rotor.
It is well known that a rotary engine may have the potential to attain a lower volume-to-power ratio as compared to many other types of engines. Additionally, the motions of the engine components in a rotary engine are substantially rotational, and the reciprocating motion associated with a piston-type engine, which may result in a large portion of frictional losses in the engine, may be substantially removed. Thus, a heat engine based on the structure of a rotary engine may have the potential to minimize the frictional losses. For these and other reasons, a rotary-type heat engine will first be employed to illustrate the embodiment of the present invention, although other types of heat engines such as a piston type will have their own advantages and are equally important to this invention.
Additionally, because of the nature of heat acquisition by the working fluid through a heat exchanger, a heat engine may face a serous dilemma. To receive a larger amount of heat from a heat source for an increased power output of the engine, a lower working fluid temperature at the end of compression is preferred, which may demand a lower compression ratio. However, this lower compression ratio may result in a lower thermal efficiency of the heat engine, which may reduce the energy utilization rate from the heat source and at the same time also have a negative effect on the power output of the engine.
A refrigerator is a system that lowers the temperature of a space or substance and then maintains that lowered temperature, and its operation may be considered the reversed operation of a heat engine. Historically, the predominant type of refrigeration system is the vapor-compression refrigeration system. However, it is well known that many refrigerants used in vapor-compression refrigeration systems may have a negative environmental impact on global warming. A refrigeration system using a gas as the working fluid without the change of phase is called the gas refrigeration system and operates on a gas refrigeration cycle. Regardless of a closed-cycle configuration or an open-cycle configuration, a gas refrigeration cycle may include three essential processes. The first process is the compression process in which the intake working fluid is compressed to a higher temperature normally above the ambient temperature, consuming an amount of work from a power source. The second process is the heat removal process in which an amount of heat is transferred from the working fluid to a heat sink, normally the ambient, due to its above-ambient temperature after the compression, lowering its temperature. The third process is the expansion process in which the working fluid returns an amount of work to the system during the expansion and at the same time further lowers its temperature, normally to a temperature below the ambient temperature as the cooled working fluid output. Similar to the discussions above related to a heat engine, a preferred gas refrigeration cycle may have the characteristics of an increased heat removal rate or duration without increasing the number of strokes per discharge stroke of the cooled working fluid in a cycle. Additionally, cooling the working fluid during the compression process could also significantly increase the efficiency of the refrigerator.
As may be known to those skilled in the art, a gas refrigeration system may have the disadvantages of an increased size and a higher mechanical frictional losses as compared to a vapor-compression refrigeration system utilizing evaporation/condensation phase-change processes for heat absorption and removal. As such, a rotary structure may be particularly attractive to a gas refrigeration system because of its potential of having a smaller volume to cooling-load ratio as well as lower mechanical friction losses. For these reasons and others, a rotary type of gas refrigeration system will first be employed to illustrate the embodiment of the present invention, although other types of gas refrigeration systems such as a piston type will have their own advantages and are equally important to this invention.